In the preparation of magnetic recording materials, such as for magnetic disks, it has been common to use magnetic particles, like Fe.sub.2 O.sub.3, dispersed in a binder mixture to form the magnetic recording material. A dispersion is usually formed by milling the ingredients together for an extended period of time in an effort to thoroughly coat the magnetic particles with the binder ingredients and to break up collections or aggregations of such particles. Magnetic particles of this type tend to cling together and it is desirable to reduce or eliminate this aggregation of particles in order to produce smaller effective magnetic particle sizes for higher density magnetic recording. The degree of uniform dispersion of the magnetic particles in the binder is an important factor in determining the final quality of the magnetic coating, as measured by the parameters of surface smoothness, orientation ratio, signal-to-noise ratio, linearity, modulation noise, coercive force and wear properties.
The milling operation described above is not always totally effective in separating the magnetic particles and causing them to remain separated until the magnetic coating material has been applied to a substrate, with the result that some aggregation of the magnetic particles does occur in the finished magnetic coating.
Surfactant materials have been applied to the magnetic particles in an effort to keep them apart, but because of the magnetic attraction between these particles, the use of surfactants alone has not been satisfactory in preventing deterioration of the dispersion with time.
It has been proposed in the prior art to provide a coating of amorphous material, such as amorphous silica, on articles of different shapes. One example of this is shown in U.S. Pat. No. 2,885,366, Iler, in which the articles to be coated are placed in a water-based dispersion having a pH of approximately 9 or higher, and silica is added thereto to coat the articles with a layer of amorphous silica. This patent does not teach the use of silica particles uniformly distributed over the surface of the coated article, nor the control of the size of the silica particles controlled in relation to the size of the particles to be coated.